In recent years, stem cell biology research has advanced, and strongly suggested the existence of cancer stem cells in solid tumors or hematological tumors. These cancer stem cells are found as small subsets, also called side populations (SPs), in solid tumors. This population, in order to maintain the tumor mass, gives rise to many cancer cells, while preserving cancer stem cells that retain their anaplastic abilities exclusively by self-renewal. The tumors may be depleted efficiently of such cancer stem cells by targeting regulatory factors specific for the cancer stem cells. The development of such therapeutic drugs can lead to the radical cure of cancers. Unfortunately, these critical regulatory factors are difficult to identify, even if cancer stem cells derived from the tumors of cancer patients are collected or if samples of cancer cell lines are collected. Due to the heterogeneity of their fractions, neither can cancer stem cell-specific factors be identified nor can their inhibitors be searched for. Hence, 100% pure cancer stem cells, if prepared, would contribute to a great progress in the above research.
The cancer stem cells are defined by (1) self-renewal, (2) pluripotency, and (3) the ability to initiate cancer of the same phenotypes as parental cancer phenotypes when transplanted to immunodeficient mice. For the sake of convenience, the term “cancer stem cells” is defined as cancer cells having the ability to self-renew (capable of dividing into malignant tumors and phenotypically diverse tumor cell populations). Many tumors are derived from single cells that have grown in vivo and have been transformed into cancer stem cells having tumor initiating abilities. The origin of the cancer stem cells, however, has not yet been elucidated.
The cancer stem cells can be identified on the basis of the expression of various cell surface markers and recovered from the tumor mass or cancer cell line using a cell sorter. The cancer stem cells collected in this way, however, are heterogenous populations. These cells, except only a small number of cells, do not possess the characteristics of pure cancer stem cells and thus, cannot be regarded as cancer stem cells in the strict sense of the term.
Some solid tumors contain specific subpopulations termed side populations (SPs), which have acquired the functional properties of cancer stem cells. These cells are able to initiate the entire tumor through self-renewal or differentiation. In the cell populations of solid tumors, cells having certain cell surface markers are known to possess the characters of cancer stem cells. For example, in brain tumor (glioblastoma), prostate cancer, and colon cancer, CD133+ cells have been shown to be cancer stem cells either by experiments on the heterotransplantation to immunodeficient mice of CD133+ cells and CD133− cells separated from these cancers, or in view of their capacity for in vitro self-renewal and growth or differentiation (Non Patent Literatures 1 to 3).
Also, in breast cancer, fractions with high expression of CD44 and low expression of CD24 (CD44(+)CD24(−/low)) have been shown to have the properties of cancer stem cells (Non Patent Literature 4).
Unfortunately, these methods which depend on cell surface markers for separating cancer stem cells are less likely to separate 100% cancer stem cells. In addition, the cells thus separated are very difficult to maintain or amplify. Against this backdrop, attempts have been made to artificially prepare cancer stem cells.
Miyoshi et al. have recently reported that the expression of reprogramming factors in various gastrointestinal cancer cells (cancer cell lines) successfully induced cancer stem cells (Non Patent Literature 5).
These cells, however, were already transformed (malignantly altered or cancerated) and acquired many gene mutations or chromosomal abnormalities. The cells therefore did not constitute homogeneous cell populations and thus seem to be unsuitable for drug screening or biomarker research. Thus, these cells are rarely defined as pure cancer stem cells.
In this connection, Yamanaka et al. have shown that induced pluripotent stem cells (iPS cells), which are similar to embryonic stem cells (ES cells), can be generated from human fibroblasts by the transfer of four genes Sox2, Oct4, Klf4, and c-Myc using retrovirus vectors (Non Patent Literature 6).
Nonetheless, these four genes have not yet been transferred to immortalized human cells. In addition, no previous report has shown that the transfer of these four genes causes not only the reprogramming of cells but also their canceration to thereby form human cancer stem cells.